1. Field of the Invention
The present invention relates to color picture tube apparatuses, and in particular to electrodes that constitute main lenses for focusing a plurality of electron beams on a screen.
2. Related Art
Generally, a color picture tube apparatus has an envelope formed from a panel and a funnel joined to the panel, and displays color images by emitting three electron beams from an electron gun disposed in a neck of the funnel, onto a phosphor screen formed opposite a shadow mask on an inner surface of the panel, while scanning horizontally and vertically, the three electron beams being deflected by horizontal and vertical magnetic deflection fields generated by a deflection yoke mounted to the outside of the funnel.
The magnetic fields of the deflection yoke used in the above color picture tube apparatus generally have a self-convergence structure to focus the three electron beams on the screen, and as a result the horizontal and vertical magnetic deflection fields are distorted into a pin-cushion shape and a barrel shape, respectively. The three electron beams that pass through the magnetic deflection fields thus undergo divergent action horizontally and focusing action vertically.
When the electron beam trajectory is lengthened due to an increase in the deflection angle, astigmatism becomes pronounced because of these magnetic self-convergence fields, particularly in a vicinity of the phosphor screen surface, and horizontal resolution is reduced as a result of the electron beam spots becoming a flattened, oblong-shape along a major axis in the horizontal direction when viewed in cross-section. This problem has been accentuated in recent years as panels become flatter and deflection angles increase.
Thus, in order to describe a high-resolution image on a phosphor screen, it is first necessary to horizontally reduce the spot diameter using the electron gun.
A known technique that attempts to do this involves applying a dynamic voltage to a focusing electrode structuring the electron gun. According to this technique, a voltage that increases with increases in the deflection angle is applied to a focusing electrode positioned closest to and facing a final electrode, and as a result, the action by the main lens electric field weakens as the deflection angle increases, astigmatism is corrected, and the shape of the beam spot is controlled.
In an application of this dynamic voltage technique disclosed in Japanese patent no. 3,040,272, attempts are made to minimize the applied dynamic voltage by adjusting the shape and orientation of beam through holes in the electrodes, and regulating the conditions under which a voltage is applied to the electrodes.
As an aside, generally the fewer spherical aberrations there are in the main lens electric field of the electron gun, the greater are the achievable reductions in the spot diameter in a color picture tube apparatus. Given an angle of incidence of the electron beams to the main lens electric field of α, the most improved spherical aberration of the main lens electric field can contribute a spot diameter δ of:δ=(M·CsP·α3)/2
Here, M is a lens magnification, and CsP is a spherical aberration coefficient. As can be inferred from this equation, weakening the lens action by the main lens electric field allows for reductions in spherical aberration. In other words, by effectively increasing the lens diameter resulting from the main lens electric field, it is possible to reduce the spot diameter on the phosphor screen.
The OLF (over-lapping field) lens disclosed in Japanese examined patent application publication 2-18540 is an example of technology that realizes this idea by way of the electrode configuration. This configuration is shown in FIG. 13.
As shown in FIG. 13, the main electrodes are constituted by a focusing electrode 101 and a final accelerating electrode 102 provided with a gap therebetween in a tube axis direction, and a shield cap 103 connected to final accelerating electrode 102.
Focusing electrode 101 and final accelerating electrode 102 are formed respectively from (i) tubular circumferential electrodes 101A and 102A, each of which has a horizontally wide, flattened tube-shape, and encompasses the three electron beams, and (ii) field-correction electrode plates 101B and 102B, each of which is set back from the facing edges of the tubular circumferential electrodes, and has three holes 101B1, 101B2, 101B3 and 102B1, 102B2, 102B3, respectively, opened therein to allow the electron beams to pass through vertically. These field-correction electrode plates 101B and 102B generate three main lens electric fields corresponding to the three electron beams.
By setting field-correction electrode plates 101B and 102B back from the facing edges of tubular circumferential electrodes 101A and 102A in focusing electrode 101 and final accelerating electrode 102, respectively, the high potential of final accelerating electrode 102 is allowed to incur deep into focusing electrode 101, and the low potential of focusing electrode 101 is allowed to incur deep into final accelerating electrode 102. As a result, the lens diameter resulting from the main lens electric fields is effectively enlarged, and the spot diameter on the phosphor screen can be reduced.
When applying dynamic voltage technology to an electrode configuration structuring an OLF lens, and seeking furthermore to minimize the dynamic voltage, it is difficult to optimally design the beam through holes to satisfy all of the various requirements using a method that involves the adjustment of the shape/orientation of the beam through holes and the regulation of the voltage applied to the electrodes. Realizability is thus poor given these design restrictions.